Molecular Simulation of H₂ Loss by Dissolution in Caprock Water-Saturated Nanopores under the Nanoconfinement Effect for Underground Hydrogen Storage

Hydrogen has been regarded as an important clean energy source in recent years. The depleted reservoir has been recognized as an economic geological site for hydrogen storage as a result of its large storage capacity. However, the caprock sealing safety of the reservoir must be considered before underground hydrogen storage. To investigate the hydrogen loss in the caprock, we have studied the H₂ dissolution in the caprock water-saturated kaolinite nanopores using molecular simulations in this work. The H₂/CH₄ mixture dissolution in the caprock nanopores was also studied to investigate the effect of the CH₄ cushion gas on the H₂ dissolution. The results showed that the H₂ dissolution can increase by up to 27 times the bulk solubility under the impact of nanoconfinement enhancement, and the gas loss through the caprock is mainly by the smaller water-saturated pores. The maximum solubility of H₂ and CH₄ occurs at the 0.55 and 0.6 nm pores, respectively. For the 0.5–0.55 nm pores, the dissolved H₂ density exceeds CH₄ at the H₂ fraction above 50%. As the pore size increases to 2.0 nm, the H₂ and CH₄ solubility is comparable as the nanopore confinement effect gradually becomes weak. However, as the H₂ fraction increases to 80%, the H₂ dissolution dominates over CH₄, even in the 2.0 nm pore. With the increase of the pore size, the water molecules gradually occupy the strong adsorption sites near the surfaces; thus, the gas molecules start to accumulate in the pore middle. The cushion gas of CH₄ adopts various molecular angles in the pores, which are 50° and 130° in the 0.55 nm pore and tripod and inclined angles of 70° and 110° in the 1.0 nm pore. However, the dissolved H₂ molecules are always perpendicular toward the pore surface, which is not affected by the pore size and water molecules. Under the influence of brine, the solubility of H₂ and CH₄ decreases and the impact of brine is more noticeable in larger pores. The high brine salinity helps to reduce the H₂ loss in the caprock nanopores.